1. Field of the Invention
The present invention relates to a vibration wave driving apparatus, such as an ultrasonic motor, and to a vibration member for use in the vibration wave driving apparatus.
2. Description of the Related Art
An ultrasonic motor (vibration wave driving apparatus) may be employed as, e.g., a driving source for a camera lens. Two types of ultrasonic motors include one having a ring-shaped vibration member and another one having a bar-shaped vibration member.
FIGS. 13A and 13B illustrate a conventional bar-shaped vibration member; specifically, FIG. 13A is a longitudinal sectional view of the vibration member, and FIG. 13B shows a vibration mode of the vibration member in the radial (R) direction. FIG. 14 shows a bar-type ultrasonic motor using a vibration member differing in construction from the vibration member of FIG. 13. FIG. 15 shows a bar-type ultrasonic motor using the vibration member of FIG. 13.
Referring to FIG. 13, the vibration member comprises a first elastic member 1 made of a metal, a second elastic member 2 made of a metal, a multilayered piezoelectric device (or a laminate of single-plate type piezoelectric devices) 3 serving as an electro-mechanical energy transducer, a shaft 4 having a step 4a formed substantially in its central area with a larger outer diameter than other areas and having threaded portions at opposite ends thereof (not shown), and a nut 5. The multilayered piezoelectric device 3 and a flexible printed circuit board (not shown) are disposed between the two elastic members 1 and 2. Using those parts, the vibration member is assembled as follows. The shaft 4 is inserted so as to penetrate through hollow central portions of the first elastic member 1, the multilayered piezoelectric device 3, the flexible printed circuit board, and the second elastic member 2, until the step of the shaft 4 abuts against the first elastic member 1. The nut 5 is screwed and fastened over the threaded end of the penetrating shaft 4 so that the multilayered piezoelectric device 3 is firmly fixed between the two elastic members 1 and 2 under a predetermined compressive force.
The vibration member in the ultrasonic motor of FIG. 14 utilizes an alternative structure including a shaft 4 in the form of a bolt. A laminate of single-plate type piezoelectric devices is firmly sandwiched between the first elastic member 1 and the second elastic member 2 by screwing a threaded portion formed substantially in an axially central area of the shaft 4 with a threaded portion formed at an inner periphery of the first elastic member 1.
In FIGS. 14 and 15, a rotor 7 has a structure in which a spring ring contacts an upper surface of the first elastic member 1, where the spring ring has a small contact width and appropriate resiliency, the spring ring is disposed below a main rotor ring, and a distal end surface of the spring ring is positioned in contact with a frictional surface of the vibration member. The other surface of the rotor 7, on a side opposite the spring ring, has a projection (or a recess) formed thereon (therein) for mating a recess (a projection) of a gear 8 that is rotated together with the rotor for transmitting a motor output. The gear 8 is fixedly positioned in the thrust direction of the shaft 4 by a flange 10 for mounting the motor, and a pressing spring 15 for imparting a pressing force to the rotor 7 is disposed between the gear 8 and the rotor 7. A ring bearing 9 is provided between the gear 8 and the flange 10, to prevent rotation. A nut 11 is screwed over a threaded distal end portion of the shaft 4 for fixing the flange 10 in place.
Electrodes of the multilayered piezoelectric device 3 (or the laminate of single-plate type piezoelectric devices) are divided into two electrode groups. When AC voltages having different phases are applied to the respective electrode groups from a power supply (not shown), the vibration member is excited with two modes of bending vibrations having orthogonal displacements, as shown in FIG. 13B (FIG. 13B shows one mode of the vibration displacements; the other mode of the vibration displacements occurs in a direction perpendicular to the drawing sheet). By adjusting the phases of the applied voltages, the two modes of vibrations can be provided with a 90-degree phase difference in time. As a result, the bar-shaped vibration member can be excited with gyrating motions in such a manner that the vibration member rotates about its axis.
Consequently, an elliptic motion is developed on the upper surface of the first elastic member 1 which is in contact with the rotor 7. The rotor 7 pressed against the wear-resistant surface member of the first elastic member is thus frictionally driven, whereby the rotor 7, the gear 8 and the pressing spring 15 are rotated as a unit in opposed relation to the first elastic member.
FIG. 15 illustrates a modification of the ultrasonic motor of FIG. 14. This version has a simplified structure that reduces the cost of manufacture. In the vibration member of FIG. 14, because a lower end portion of the shaft 4 has a large diameter, the shaft must be cut from a large-diameter material. Therefore, a longer relative working time is required and the material cost is wastefully increased. Another disadvantage is that, since the shaft has a large diameter difference between an upper portion and a lower portion thereof, the vibration member of FIG. 14 is not suitable for plastic working, such as forging, which is relatively inexpensive; rather, the shaft must be formed by machining.
The ultrasonic motor of FIG. 15 is free from those disadvantages. Namely, the ultrasonic motor of FIG. 15 reduces the cost of manufacturing by forming a shaft into a shape obtainable by forging.
The bar-type ultrasonic motors of FIGS. 13 to 15 are much smaller than ring-type ultrasonic motors, and individual parts are simpler in shape than those of the ring-type ultrasonic motors, thereby minimizing the working cost of the parts.
In order to further reduce the motor size, a proposal for shortening the motor length has also been made.
With a reduction of the motor size, however, the part size is also reduced, which is disadvantageous from the standpoint of part strength. Assuming, for example, the case of manufacturing a motor using a vibration member that has a size reduced to ½ of the vibration member of FIG. 13, the diameters of the elastic members and the shaft are reduced to ½ and therefore the area of a contact surface between the elastic member 1, 2 and the piezoelectric device 3 is reduced to ¼.
To keep the surface pressure in such a contact surface equal to that before the size reduction, the cross-sectional area (i.e., the tensile strength) of the shaft can be reduced to ¼ without problem because the fastening force required for tightening the nut against the shaft is also reduced to ¼. When the elastic members are fastened using screws, as with the vibration member of FIG. 13, the required fastening torque also becomes ¼ on condition that the fastening torque is proportional to the compressive force in the axial direction of the shaft. However, since the maximum shearing stress τ generated in the shaft is expressed by τ=16T/πd3 (where T is the fastening torque and d is the shaft diameter), it becomes twice that generated in the shaft having the original size. In other words, the strength of the shaft is reduced to ½ if the same material is used. Particularly, where one end portion (upper half portion) of the shaft has a smaller diameter as shown in FIG. 13, the following problem occurs. In fastening the nut 5 with a jig 24 as shown in FIG. 17, the shaft 4 is fixed using a jig 25 and prevented from turning. However, when the nut is fastened with the jig 25 gripping the smaller-diameter portion of the shaft 4, a torsional rupture is apt to occur in the smaller-diameter portion. Hence, the fastening torque cannot be applied at a sufficient level.
Also, for a vibration member having a size larger than a certain value, as shown in FIG. 16, the vibration member can be assembled by applying a prestress (indicated by P in FIG. 16) from above while supporting the step of the shaft 4 with a jig 23 or the like, holding the shaft 4, the elastic members 1, 2 and the piezoelectric device 3 together in a fixed condition, and then fastening the nut 5 with a jig 22 fitted over the nut 5. For a vibration member having a small size (with the shaft diameter of, for example, not more than 2 mm), however, even a space for allowing insertion of the jig 23 cannot be ensured.
On the other hand, when such a Langevin vibration member as shown in FIG. 13 has a structure wherein the first elastic member 1 is formed with a threaded portion similar to that of the nut 5 and the piezoelectric device is fastened while gripping the nut 5 and the first elastic member 1, a difficulty arises in setting, to a predetermined position, the relative position of a group of the elastic members 1, 2, the piezoelectric device 3 and the nut 5, which are fastened into a single unit, with respect to the shaft 4 in the thrust direction thereof.
More specifically, when the nut 5 is rotated and fastened while the first elastic member 1 is fixedly gripped, the shaft 4 is also rotated together with the nut 5 by frictional forces produced in the threaded portion of the nut 5. Therefore, the shaft 4 is moved upward, as viewed in FIG. 13, relative to the first elastic member 1. In other words, as the nut 5 is rotated and fastened, the first elastic member 1 is moved farther away from the flange 10. Because the amount by which the shaft 4 is moved differs depending on each case, it is difficult to always arrange the shaft 4 and the first elastic member 1 in the same relative position. This leads to a difficulty in setting a constant rotor pressing force in the structure wherein the rotor pressing force is defined/set depending on the distance between the flange 10 fixed to the shaft 4 and the first elastic member 1.